When used in nuclear reactors, graphite becomes contaminated with various radiotoxins, which are formed by the neutron activation of impurities contained in the graphite and/or by the neutron activation of the ambient atmosphere and/or by nuclear fission. These radiotoxins pose a particular problem for disposal if these are highly volatile, such as tritium (3H) for example, are long-lived and volatile, such as 36Cl, or emit highly penetrating radiation, such as 60Co.
In addition, the carbon that is present in the graphite itself, having an atomic weight of 13, is activated to form radiocarbon (14C). The capture cross-section for this reaction is only 0.0009 barns; however this should not be disregarded because of the significant concentration of 13C of 1.11%. In addition, 13C is generated by the neutron capture of 12C (0.0034 barns) during exposure to radiation and thus also increasingly contributes to the formation of 14C. Furthermore, radiocarbon is generated by the neutron activation of nitrogen having an atomic mass of 14. At 99.64%, 14N accounts for the majority of naturally occurring nitrogen. The cross-section for neutron capture with subsequent emission of a proton is 1.81 barns. In addition, radiocarbon forms via the oxygen isotope having an atomic mass of 17, which occurs naturally in an abundance of 0.039%, by way of neutron capture (0.235 barns) with subsequent emission of an alpha particle. In the neutron field, nitrogen and oxygen can be either a constituent of the reactor atmosphere or part of chemical bonds in reactor materials.
Radiocarbon differs from the stable isotopes of carbon only in the atomic mass thereof. Thus, this basically exhibits the same chemical behavior as the remaining isotopes of carbon. This is also reacted in biological processes as stable carbon and is not recognized as a foreign substance. The release thereof into the biosphere should therefore be avoided. This is also the reason why the threshold values for the final storage and for the potential release of radiocarbon are extremely stringent.
The radiotoxins, and here 14C in particular, form only a fraction of several ppm (parts per million) in the overall mass of the graphite, however these are distributed substantially homogeneously over the entire volume thereof, so that the entire volume is considered radioactive waste, which in some countries is categorized as intermediate-level waste (ILW) or, due to the half life of 5730 years, as long-lived low-)evel waste (LLLW). Because final storage capacities are scarce and expensive, a need therefore exists for selectively removing radiotoxins from the graphite and concentrating the same in such an amount that the remaining graphite can either be categorized as low-level waste (LLW), or that even safe levels are measured and this can be reused.
A method is known from DE 10 2004 036 631 A1 for selectively removing radiocarbon by way of a chemical reaction at elevated temperatures. To this end, a gaseous corrosion medium is loaded against outer and inner surfaces of the graphite, wherein only a few percent of the graphite corrode, but a large portion of the radiocarbon is released. The method takes advantage of the realization that the majority of radiocarbon has become concentrated in the vicinity of the inner surfaces in the pore system of the graphite, since the radiocarbon is assumed to be essentially created by the activation of adsorbed nitrogen or oxygen in the pore system of the graphite, and thus to be oxidized with preference.
Improvement potential is considered to exist to the effect that a significant amount of radiocarbon and/or other radioactive isotopes remains in the waste after the method is carried out.